1. Field of the Invention
The present invention relates to a heat treatment process for effecting a film formation, oxidation, impurity dispersion, etc., on a surface of an article to be treated such as a semiconductor wafer.
2. Description of the Related Arts
Among film forming processes for the production of semiconductor devices is a process called a reduced pressure CVD (chemical vapor deposition). This process comprises the steps of introducing a treatment gas into a reaction tube while reducing the pressure within the reaction tube, and forming a film on the surface of a wafer through a chemical vapor-phase reaction. This process is advantageous in that a multiplicity of wafers can be treated at one time because reduced pressure permits the treatment gas to fully enter narrow gaps between adjacent wafers.
As such a batch type film deposition apparatus, much use is recently made of a vertical type heat treatment apparatus because of its small amount of entry of the environmental air. In a conventional reduced pressure CVD process using a vertical type heat treatment apparatus in the case of, for example, forming a polysilicon (polycrystalline silicon) film, a multiplicity of wafers are first mounted on a wafer boat, the reaction tube is filled with a nitrogen gas atmosphere, and the temperature within the reaction tube is raised up to a treatment temperature, for example, of the order of 620.degree. C. The wafers are then loaded into the reaction tube together with the wafer boat. After hermetically closing an opening at the lower end of the reaction tube by means of a closure member, the reaction tube is evacuated for about ten (10) minutes to obtain an atmosphere of a predetermined vacuum. Then, a monosilane (SiH.sub.4) gas is supplied into the reaction tube for twenty (20) minutes, for example, to form a polysilicon film on the surfaces of the wafers. Subsequently, nitrogen gas is supplied into the reaction tube for about ten (10) minutes to return the interior of the reaction tube to normal pressure atmosphere. Then, the wafers resting on the wafer boat are unloaded from the interior of the reaction tube.
Among processes for manufacturing semiconductor devices there are an oxidation treatment in which a silicon surface is oxidized at high temperature to obtain an insulating film (oxide film), and a dispersion treatment in which a silicon layer whose surface is implanted with impurities is heated to thermally disperse the impurities into the silicon layer.
As a heat treatment apparatus for carrying out such oxidation or dispersion treatment, use of a vertical type furnace, instead of a horizontal type furnace, is increasing because of its small amount of ingress of the air. In a conventional example of the oxidation treatment by use of a vertical heat treatment apparatus, a multiplicity of wafers are mounted in a wafer boat, the reaction tube is filled with a mixed gas atmosphere of N.sub.2 (nitrogen) gas and O.sub.2 (oxygen) gas, and the interior of the reaction tube is heated up to a temperature of the order of 800.degree. C., which is lower than the treatment temperature. The wafers are then loaded into the reaction tube together with the wafer boat. After hermetically closing an opening at the lower end of the reaction tube by means of a closure member, the treatment temperature within the reaction tube is raised up to, for example, 900.degree. to 1100.degree. C. at a rate of 10.degree. C./min. This temperature is maintained for a predetermined period of time to form an oxide film on the surface of the silicon layer. Afterwards, the temperature within the reaction tube is lowered to 800.degree. C. at a rate of about 2.degree. C. The wafers resting on the wafer boat are then unloaded from the interior of the reaction tube.
In the impurity dispersion treatment, wafers containing impurity ions, for example, arsenic (As) ions implanted into the surface of the silicon layer of the wafers are held under a heated atmosphere at 900.degree. to 1000.degree. C. and under a N.sub.2 atmosphere, to thereby disperse the As ions into the silicon layer. Except for using a different gas atmosphere within the reaction tube, a vertical type heat treatment apparatus can be used in the same manner as in the case of the oxidation.
The conventional heat treatment processes described above involve the following problems.
In the case of film formation treatment, environmental air tends to be drawn in through the opening at the lower end of the reaction tube and enter the interior of the tube although the reaction tube is filled with nitrogen gas at the time of the wafer loading. Because the temperature within the reaction tube is raised up to a treatment temperature of about 620.degree. C., the growth of the natural oxidation film is promoted if the wafers are brought into contact with the oxygen under such high temperature. Therefore, when the reduced pressure CVD is carried out, a natural oxidation film will be formed on an interface between the polysilicon and an underlayer such as monocrystalline silicon film, and the contact resistance between the adjacent films will increase. Since the natural oxidation film is extremely thin, it does not much influence the characteristics of the conventional semiconductor devices. However, because of high integration of the devices in recent years and resultant miniaturization in the width of pattern, the characteristics of the semiconductor devices are much influenced by increased contact resistance caused by the above-described natural oxidation film.
Furthermore, due to a high temperature atmosphere to which the wafers are abruptly subjected during the loading, radiation from the heater surrounding the reaction tube causes the peripheral portions of the wafers to be subjected to a high temperature than the central portion thereof. The resultant thermal strain will cause cracks called "slips" on the surfaces of the wafers.
The treatments to be carried out by the vertical type heat treatment apparatus include a film formation treatment for forming a metal-silicon film by heating a wafer having a metal film such as a Ti film formed on the surface of the polysilicon layer or monocrystalline silicon layer. The above problems also apply to this case because the loading into the reaction tube is effected under a treatment temperature of, for example, 550.degree. to 600.degree. C.
In the case of the oxidation treatment, due to the loading of cool wafers and wafer boat supporting them into the reaction tube, the temperature within the reaction tube is temporarily lowered to a temperature of the order of 700.degree. C. from 800.degree. C. However, the wafers are abruptly subjected to such high temperature during the loading, as in the case of the film formation treatment, the peripheral portions of the wafers are caused to be higher in temperature than the central portion thereof by virtue of the radiation heat from the heater around the reaction tube. The resultant thermal strain therefore causes a shearing deformation called "slip" and warping.
If O.sub.2 comes into contact with the surface of the silicon layer at a lower temperature than the predetermined temperature, there will grow on the surface an oxide film having poor film properties, that is, having a low pressure-resistance. Thus, if the wafers are subjected to a range of temperatures lower than the predetermined treatment temperature for a long period of time with a slow heating rate of 10.degree. C./min, there will appear an oxide film having poor film properties, underlying the original oxide film having good film properties, which results in deteriorated film properties as a whole. Without O.sub.2 gas supplied into the reaction tube, such problems could be obviated. It is however necessary to supply O.sub.2 gas for preventing nitrification since with only N.sub.2 gas the silicon layer would be nitrified to roughen the surface.
There is a recent tendency toward the miniaturization and reduction in the thickness of the pattern of semiconductor devices. Because of this tendency, for providing a capacity insulating film, for example, of a CMOS by using the oxidation treatment, the oxide film must be significantly thinned to ensure a large capacity thereof. Thus, the intervening oxide film having poor film properties formed under an atmosphere lower than the treatment temperature will cause a reduction in the yield.
During the loading, the wafers near the top of the wafer boat are exposed to a high temperature in O.sub.2 gas atmosphere for a longer period of time than the wafers near the bottom thereof, which will result in an unevenness in the degree of the oxide film formation that depends on vertical positions of the wafers within the wafer boat. The resultant unevenness will possibly influence the characteristics of the semiconductor devices produced.
In the case of the dispersion treatment described above, with the miniaturization of the semiconductor devices, the dispersion depth of the impurities such as arsenic (As) in the silicon layer tends to become smaller to provide a shallow p-n junction. On the other hand, because the concentration profile of impurities within the silicon layer is largely influenced by the thermal history, the concentration profiles of the wafers differ depending on their respective vertical positions within the wafer boat. Consequently, if the wafer boat is abruptly loaded into or unloaded from the reaction tube at a temperature as high as 800.degree. C., the wafers may suffer generation of slips therein described above as well as possible breakage thereof. Therefore, the loading and unloading must be carried out at a slow rate. However, the closer to the top of the wafer boat a wafer is located, the longer becomes the time during which the wafer is subjected to high temperature atmosphere, and the more thermal history the wafer undergoes. As a result, the upper wafers are liable to exhibit a small impurity concentration in the vicinity of the surface and a gentle concentration gradient in its depth direction. Consequently, it becomes difficult for the wafers within the wafer boat to undergo a uniform dispersion treatment.